Speciation of Aluminum Precipitates from Acid Rock Discharges in Central Pennsylvania

Abstract

Environmental and mineralogical characteristics of precipitates from aluminum containing acid rock discharge from a location Centre County, PA, were determined using X-ray diffraction, scanning electron microscope (SEM) equipped with an energy-dispersive X-ray spectrometer, and thermogravimetric analysis. The precipitates were formed from surface seeps and from existing on-site flush box limestone treatment systems treating those surface seeps. Based on the energy-dispersive X-ray spectrometry analysis, the average Al:OH ratios of the precipitates in acid rock discharge formed in stream and in the flush boxes were 1:1.9 and 1:2.8, respectively. Thermogravimetric analysis showed that both OH− and were major constituents in the aluminum precipitates, and precipitates were mainly in the form of amorphous aluminum hydroxide and aluminum sulfate. Simulation of water chemistry with the Mineql+ model showed that the possible species of precipitates in the acid rock drainage impacted stream were amorphous Al(OH)3 and aluminum sulfate compounds, most of which are jurbanite [Al(OH)SO4]. The saturation index for these compounds indicated that the soluble aluminum concentration was controlled by jurbanite and amorphous Al(OH)3. This study suggests that the total soluble aluminum species in treated acid mine or acid rock discharges is highly influenced by sulfate levels in the stream and are in equilibrium with both amorphous Al(OH)3 and aluminum hydroxyl sulfates, jurbanite, and likely not in equilibrium with gibbsite (crystalline Al(OH)3).